RU2794744C1 - Method for increating critical heat fluxes in a fuel assembly with tubular fuel rods - Google Patents

Abstract

FIELD: increasing critical heat fluxes in a fuel assembly with tubular fuel elements.

SUBSTANCE: according to the method, the coolant is supplied to the inlet of the fuel assembly, the coolant is distributed between the inter-fuel space and the internal cavities of the fuel elements (fuel rods), the coolant is passed along the convex and concave heat-releasing surfaces of the fuel rods, heat is released in the fuel rods, the generated heat is redistributed over the thickness of the fuel rods, the generated heat is removed due to heat removal from the concave and convex surfaces of the fuel element. Moreover, the redistribution and removal of heat from the heat-removing surfaces is carried out due to the fact that the flow entering the inter-fuel space is preliminarily divided into at least two flows, fed to the convex heat-releasing surface of the fuel elements, the flow remote from the convex surface (upper flow) is swirled, the transit flow flowing along the convex heat-releasing surface of the fuel elements and the swirling flow interact, the flows formed by the swirlers and the flows passing along the axis in the gaps between the swirlers interact. Next, the swirling flow that occurs when the coolant passes through the holes in the wall of the fuel element, and the transit flow flowing along the convex heat-releasing surface of the fuel element interact.

EFFECT: possibility of achieving a high energy density of the fuel assembly.

1 cl, 8 dwg

Description

The invention relates to energy and can be used in devices for heating water, such as nuclear power plants.

There is a known method for increasing critical heat fluxes (CFT) in a fuel assembly, which consists in supplying a coolant to the inlet of the fuel assembly, passing the coolant along the convex heat-releasing surfaces of the fuel elements (fuel rods), releasing heat in the fuel rods, removing heat from the convex heat-releasing surfaces of the fuel rods (F.Ya. Ovchinnikov, VV Semenov Operational modes of VVER (M. Energoatomizdat. 1988, p. 149, [1]).

The main disadvantage of the TVS operation method is that it does not allow to obtain a high energy intensity of the assembly. The latter is due to the following. Since the reserves before the heat transfer crisis are determined by the average mixed assembly parameters (over the cross section), the calculated values of critical heat fluxes (CTF) can differ significantly from those that occur in a real situation. This is due to the fact that fuel assemblies always contain unheated elements (FEL, CPS rods), the presence of which leads to a significant decrease in CHF, compared with those CHF that occur in fuel assemblies with thermally hydraulically equivalent cells. Currently, there are practically no methods for calculating the CHF in fuel assemblies with thermally unequal cells. The methods used to estimate reserves before the crisis are imperfect and determine the CFT with a large error. (Pometko R.S., Boltenko E.A. et all. The critical Heat Flux in WWER Fuel Subassembly Model with Nonuiform Crossectional Parameters Distubution / NURETH -8, September 30-October, 1997, Japan, [2]). In order to avoid emergency situations associated with an incorrect determination of the CHF, the allowable values of the CHF (reserves before the heat transfer crisis) in assemblies are underestimated, thereby reducing the energy intensity and, accordingly, the efficiency of fuel assemblies.

One of the reasons for the low values of CHF in fuel assemblies of this design is that the coolant over the cross section of the fuel assemblies is weakly mixed. The latter leads to the fact that in the fuel assembly cells the coolant has different parameters (temperatures, velocities, etc.). Any perturbation at the input is transmitted practically to the assembly output. In this regard, the local deterioration of the thermal-hydraulic situation in the FA cell leads to a local deterioration in the temperature regime of the fuel element (with appropriate heat fluxes) and, as a consequence, to a heat removal crisis, its failure and, accordingly, the FA failure.

In the work (RF Patent for the invention 2680175 C1, IPC ³ F28F 13/12. A method for increasing heat removal on the convex heat-releasing surfaces of heat transfer devices and a device for its implementation / E.A. Boltenko // Application No. 2016142832/28, dated 10/31/2016, (published on May 3, 2018, Bull. No. 13/ Published: February 18, 2019. Bull. No. 5, [3].) the problem of significantly increasing the intensity of heat removal and critical heat flow on convex heat-releasing surfaces is solved, which consists in separating the flow supplied to the convex heat-releasing surface by two, and spinning the upper stream. The intensification of heat removal is achieved due to the interaction of the upper swirling and transit flow moving along the convex surface.

An increase in the intensity of heat removal on a convex heat-releasing surface is also achieved due to the fact that the lower flow moving along the convex heat-releasing surface is twisted. device placed on a convex surface lies in the range 0<T _n <∞s, and the angle of twist of the lower swirl device ϕ _n is directed opposite to the angle of twist of the upper swirl device ϕ _in . An increase in the intensity of heat removal on a convex heat-releasing surface is also achieved due to the fact that the upper swirling flow, remote from the convex surface, is preliminarily divided into a number of phase-shifted flows [3].

In the work (Patent of Russia 2359346 MPK ³ G21C 3/322 (2006.01). The method of operation of the fuel assembly / E.A. Boltenko // Application No. 2006111371/06 dated 10.04.2006. Bull. No. 17. 2009 [4]) The problem of increasing the intensity of heat removal and the critical heat flux on the concave heat-releasing surface of a fuel element was solved by swirling the coolant and subsequent redistribution of heat flows over the thickness of the fuel element due to thermal feedbacks.

The closest in technical essence and the achieved result is the method of operation of the fuel assembly, which consists in supplying the coolant to the inlet of the fuel assembly, distributing the coolant between the inter-fuel space and the internal cavities of the fuel elements (fuel rods), passing the coolant along the convex and concave heat-releasing surfaces of the fuel rods, separating heat in the fuel elements, redistribute the generated heat over the thickness of the fuel element, remove the generated heat due to heat removal from the concave and convex heat-releasing surfaces of the fuel elements (Patent of Russia 2220464 MKM ³ G21С3 / 00, 3/30, 3/32. Fuel assembly / V.N. Blinkov , E.A. Boltenko // Application No. 2002104121 dated February 20, 2002. Discoveries. Inventions. 2003. No. 36, [5]).

The main disadvantage of the method is that this method of operation does not allow to achieve a high energy intensity of the fuel assembly. For example, if the FA operation mode deviates from the nominal value, the flow rate through the FA will start to decrease and the FA will operate in off-design mode. In this case, the coolant will boil at the outlet of the assembly, i.e. assembly will run in boiling mode. In this case, both on the concave and convex heat-releasing surfaces, the so-called heat transfer crisis of the "second kind" may occur, the CHF values will decrease significantly (up to 2-3 times). Since the heat fluxes on the assembly are quite large, the achievement of a crisis will lead to a significant increase in the wall temperature (2000-3000°C) and the failure of the assembly.

A method is proposed for increasing the critical heat fluxes in a fuel assembly with tubular fuel elements, which consists in supplying a coolant to the inlet of the fuel assembly, distributing the coolant between the inter-fuel space and the internal cavities of the fuel elements (fuel elements), passing the coolant along the convex and concave heat-releasing surfaces of the fuel elements, and releasing heat in fuel elements, redistribute the released heat over the thickness of the fuel elements and remove the generated heat due to heat removal from the concave and convex heat-releasing surfaces, characterized in that the redistribution and removal of heat from the heat-removing surfaces is carried out due to the fact that the flow entering the inter-fuel space is pre-divided, according to at least two flows, they are fed to the convex heat-releasing surface of the fuel element, the flow (upper) remote from the convex surface is swirled, the transit flow flowing along the convex heat-releasing surface of the fuel element and the swirling flow are interacted, the flows formed by swirlers and flows passing through in the gaps between the swirling devices, the interaction of the swirling flow that occurs when the coolant passes through the holes in the wall of the fuel element and the transit flow is carried out.

The technical result, to which the invention is directed, is to increase the critical heat fluxes in the fuel assembly with tubular fuel elements, which is ensured by the fact that the redistribution and removal of heat from the heat-removing surfaces is carried out due to the fact that the flow entering the inter-fuel space is pre-divided, according to at least two flows are fed to the convex heat-releasing surface of the fuel element, the flow (upper) remote from the convex surface is swirled, the transit flow flowing along the convex heat-releasing surface of the fuel element and the swirling flow are interacted, the flows formed by swirlers and flows passing into gaps between the swirling devices, the interaction of the swirling flow that occurs when the coolant passes through the holes in the wall of the fuel element and the transit flow flowing along the convex heat-releasing surface of the fuel element is carried out.

The achievement of the technical result is ensured by the flow of the coolant into the inter-fuel space, preliminary separation into at least two flows of the coolant and supply to the convex heat-releasing surface of the fuel element, swirling the flow remote from the convex surface (upper), conducting the interaction of the transit flow flowing along the convex the heat-releasing surface of the fuel element, and the swirling flow, conducting the interaction of flows formed by the swirlers and flows passing in the gaps between the swirling devices, conducting the interaction of the swirling flow that occurs when the coolant passes through the holes in the wall of the fuel element and the transit flow.

In FIG. 1 and FIG. Figure 2 shows electrically heated assemblies with tubular fuel rods, in which transit and swirling flows are formed on a convex heat-releasing surface. In FIG. 1, the transit flow is formed with the help of longitudinal ribs mounted on a convex surface, the swirling flow is formed with the help of swirling devices. In FIG. 2, the transit flow is formed using spacers (wires) mounted on a convex surface, the swirling flow is formed using swirlers. In FIG. 3 shows a tubular fuel element with double-sided heat removal. In FIG. Figures 4 and 5 show diagrams of tubular fuel elements that implement the interaction of transit and swirling flows. In FIG. 6 shows the qualitative nature of the dependence of the critical heat flux on the vapor content on the convex heat-releasing surface of the annular channel.

The fuel assembly, FA, operates as follows. The coolant supplied to the inlet of the fuel assembly is distributed between the internal cavities of the fuel rods and the inter-fuel volume, washing the convex and concave heat-releasing surfaces of the fuel rods. The coolant supplied to the inter-fuel volume is preliminarily divided into at least two flows, the coolant is supplied to the convex heat-releasing surface of the tubular fuel elements, the flow (upper) remote from the convex surface is swirled. The interaction of the transit flow flowing along the convex heat-releasing surface of the fuel element and the swirling flow is carried out, the flows formed by the swirlers and the flows passing in the gaps between the swirlers are interacted, the swirling flow that occurs when the coolant passes through the holes in the wall of the fuel element and the transit flow are interacted.

The distribution of coolant flow rates occurs in accordance with the hydraulic resistance of the internal cavities of the fuel elements and the inter-fuel volume. Further, the coolant passing through the space between the fuel rods and the internal cavities of the fuel rods, due to heat release in the fuel rods, is heated to the required value and exits the fuel assemblies.

In FIG. 1 and FIG. 2 shows seven-rod assemblies with tubular fuel rods. In FIG. 3 shows a tubular fuel element. In FIG. 4, the transit flow is formed by installing (welding) bulges (spacers, wires) on a convex surface, the swirling flow is formed by installing twisting devices on the bulges. In FIG. 5 to form a transit flow, 4 longitudinal ribs are installed, on which a swirling device is welded. Longitudinal clearance, Fig. 4 and longitudinal ribs, FIG. 5 perform the following functions - firstly, they form channels for the passage of the transit flow, and secondly, they form cells that have a common boundary along the liquid with a swirling flow, which is created by a swirling device.

Due to the interaction of swirling and transit flows on a convex heat-releasing surface, the heat transfer coefficient increases significantly. In this regard, due to thermal feedback, part of the heat that was removed from the convex heat-releasing surface will be transferred to the concave surface. In the event of an emergency process (deterioration of heat transfer), all heat will be transferred from the convex surface, and the maximum temperature of the fuel element will be equal to the temperature from the side of the convex surface.

On the convex surface, sections with swirl and transit flow are located at a length of 50-100 mm. Between these sections are sections with a smooth surface 100 mm long. The twisting devices are installed with a gap relative to the convex surface of the fuel rod. The spacing of the fuel rods is made using spacer grids.

Due to the interaction of the transit and swirling flow, there is a significant increase in the heat transfer coefficient and the critical heat flow on the convex heat-releasing surface. The use of swirling and transit flows on a convex surface makes it possible to increase the critical heat flows in a fuel assembly with tubular fuel elements.

The increase in critical heat fluxes in a fuel assembly with tubular fuel rods occurs due to a number of effects. Due to the presence of a liquid boundary between the transit flow and the swirling flow, the centrifugal accelerations g* created by microvortexes are much higher than g of the main swirling flow g*>>g (d>>h), g*=U ² /r. Due to the presence of a liquid boundary, the circumferential velocities in the cells are equal to the circumferential velocity formed by the swirler 2, FIG. 4, fig. 5.

где d_вн - диаметр выпуклой теплоотдающей поверхности, Т - шаг закрутки, W - средняя скорость набегающего потока. Наличие твердых границ и жидкой границы, имеющей окружную скорость, способствует образованию микровихрей с центрами вращения в межреберном пространстве, радиус вращения которых порядка высоты транзитного потока, (зазора, высоты ребер h). Так как h=0,1÷0,5 мм, что значительно меньше d, центробежные ускорения g*, создаваемые микровихрями, значительно выше g основного закручивающего потока g*>>g (d>>h), g*=U²/r.

where d _ext is the diameter of the convex heat-releasing surface, T is the twist pitch, W is the average velocity of the oncoming flow. The presence of solid boundaries and a liquid boundary with a circumferential velocity contributes to the formation of microvortices with centers of rotation in the interfin space, the radius of rotation of which is of the order of the height of the transit flow (gap, height of the ribs h). Since h=0.1÷0.5 mm, which is much less than d, the centrifugal accelerations g* created by microvortexes are much higher than g of the main swirling flow g*>>g (d>>h), g*=U ² / r.

Due to a significant increase in g*, the effects associated with the ejection of moisture onto the convex heat-releasing surface and the suction of steam from the surface are enhanced. The interaction of the transit flow with the swirling device leads to additional flow turbulence and the release of moisture onto the heat-releasing surfaces. As a result, there is a significant increase in critical heat fluxes and intensity of heat removal on the convex heat-releasing surface.

An increase in critical heat fluxes and intensity of heat removal also occurs due to the interaction of flows formed by swirlers and flows passing in the gaps between swirlers, an increase in critical heat fluxes also occurs due to the transit flow that occurs when the coolant passes through the holes in the fuel element. In FIG. Figure 6 shows the values of critical heat fluxes on a convex heat-releasing surface as a function of vapor content for an annular channel in which the swirling and transit flows interact [6, 10]. As can be seen from FIG. 6, the interaction of transit and swirling flows on a convex heat-releasing surface leads to a significant increase in the critical heat flow.

The holes in the wall of the fuel element provide hydraulic feedback between the internal cavities of the fuel element and the space between the fuel elements. In this case, in addition to controlling the thermal feedback and redistributing the coolant at the fuel element inlet, swirl will also change the coolant flows along the length of the fuel element due to hydraulic feedback.

An example of a specific implementation. Determination of reserves before the heat transfer crisis. At present, stocks before the crisis in fuel assemblies are defined as follows, Fig. 7:

1. Based on the geometric, regime, input and other parameters, local parameters in the assembly cells or averaged parameters over the section are determined.

2. The critical heat fluxes of CHF for local parameters are determined, and reserves before the crisis are found as the ratio of CHF to the local heat flux from the surface of the fuel element.

3. From all possible margin values for various cassettes, various cells and sections along the length, the minimum is selected, which is taken as the limiting factor in terms of the level of energy release in the reactor.

This approach, based on comparing the CHF for given local parameters, with the local flow from the fuel element surface, has a certain disadvantage. The disadvantage is that the safety factor defined as q _cr /q _lok makes sense for fixed variables of local parameters and cannot be implemented both in normal core conditions and in transient or emergency situations.

In FIG. 8 shows a general view of the dependence of the critical heat flux on the vapor content q _kr (x). As you can see, with sufficiently high local stocks before the crisis, the real stock can be very small - an emergency situation is possible. Really high reserves create the appearance of large reserves of the installation (K ₃ = 3-4) in nominal modes. Meanwhile, an increase in the heat flow by 1.5–2 times leads to a crisis due to an increase in the vapor content and a sharp decrease in the CHF (the transition region of Fig. KTP) gives a more objective picture of the real possibilities of the installation.

In fuel assemblies with tubular fuel elements, where two-sided heat removal takes place, the assessment of reserves before the crisis was made on the basis of known correlations for convex (outer surface) and concave (inner fuel element surface) heat-releasing surfaces and taking into account thermal feedback. There are no experimental data on reserves before the crisis in fuel elements with double-sided heat removal. There are known computational works on the assessment of reserves before the crisis in fuel assemblies with tubular fuel elements with a different number of fuel elements [7]. Stocks before the crisis in these works are determined by the traditional method, i.e. compared to the local heat flux and CHF in this section. The reserves before the crisis amounted to 3.4 for the convex and 2.9 for the concave heat-releasing surfaces of a tubular fuel element, which is more than twice as high as the reserves before the crisis of conventional fuel rods.

In [8, 9], a method for calculating the thermal-hydraulic characteristics of a reactor facility (reactor plant) with tubular fuel elements (single-rod model) was developed, and the FUTEI (Fuel Tube with External and Internal Cooling) program was implemented based on the method. As a result of the calculation, the following quantities are determined: coolant flow in the annular slot G _to and pipe G _tr , pressure losses in the channel, temperatures of the concave t _stvp and convex t _stvp heat-releasing surfaces, the maximum temperature of the fuel element at the nominal mode t _m . The calculated water flow rates in the annular slot and pipe, as well as the heat flow profile on the convex and concave heat-releasing surfaces, serve as the initial data for calculating the reserves before the heat transfer crisis on the corresponding heat-releasing surfaces. Based on the model of an equivalent annular channel using the FUTEI program, the reserves before the heat transfer crisis for a tubular fuel element were calculated. Based on the calculation, it is shown that the reserves before the heat transfer crisis for a tubular fuel element significantly (more than 2 times) exceed those for a rod fuel element. It is shown that the calculation results are comparable with those obtained in the calculations in assemblies. On the basis of the FUTEI program (single-rod model), the calculation of reserves before the heat transfer crisis was performed in relation to the 7-rod assembly. Parameters of a single-rod model: concave surface - d _tr - pipe with a diameter of 8 mm, annular slot convex surface - d _VP pipe with a diameter of 12 mm, an annular gap of 1.5 mm. Heating length 3 m. Operating parameters: mass velocity 1000 kg/m ² s, pressure P=12 MPa, T _in =290 C. Nominal power 67 kW. th q _max / q _min \u003d 1.8. Heat flux q _cp \u003d 0.36 MW / m ² .

The calculation of reserves before the heat transfer crisis for a single-rod model is performed. Concave surface-pipe K _C =3.6; The convex surface is an annular gap K _z \u003d 5.4.

The calculation of reserves before the heat transfer crisis for a single-rod model, taking into account the interacting swirling and transit flows, was K _z =3.6 for a concave surface - a pipe, K _z = 7.2 for a convex surface, taking into account the interaction of transit and swirling flows.

Thus, the proposed technical solution makes it possible to increase the critical heat fluxes in the assembly with tubular fuel elements and, accordingly, to increase the reserves before the heat transfer crisis.

An increase in the critical heat flow is achieved due to the interaction of the transit flow flowing along the convex heat-releasing surface of the fuel element and the swirling flow, the interaction of the flows formed by the swirlers and the flows passing in the gaps between the swirlers, the interaction of the swirling flow that occurs when the coolant passes through the holes in the wall of the fuel element and the transit flow.

Literature

1. F.Ya. Ovchinnikov, V.V. Semenov Operational modes of VVER. M. Energoatomizdat. 1988, p. 149.

2. Pometko R.S., Boltenko E.A. et all. The critical Heat Flux in WWER Fuel Subassembly Model with Nonuiform Crossectional Parameters Distubution / NURETH-8, September 30-October, 1997, Japan.

3. RF patent for the invention 2680175 C1, IPC ³ F28F 13/12. A method for increasing heat removal on convex heat-releasing surfaces of heat transfer devices and a device for its implementation / E.A. Boltenko // Application No. 2016142832/28, dated October 31, 2016, published on May 3, 2018, Bull. No. 13/ Published: 18.02.2019. Bull. No. 5).

4. Russian patent 2359346 MIIK ³ G21С 3/322 (2006.01). The method of operation of the fuel assembly / E.A. Boltenko // Application No. 2006111371/06 of 04/10/2006. Bull. No. 17. 2009

5. Russian patent 2220464 MKM ³ G21С 3/00, 3/30, 3/32. Fuel assembly / V.N. Blinkov, E.A. Boltenko // Application No. 2002104121 dated February 20, 2002. Discoveries. Inventions. 2003. No. 36.

6. Boltenko E.A. Investigation of the heat transfer crisis on the heat-releasing surfaces of annular channels with swirl and transit flow / Teploenergetika, 2016, No. 10 p. 42-47.

7. Zhao J., No. H.S., Kazimi M.S. Mechanical Analysis of High Power Internally Cooled Annular Fuel // Nucl. Technology, 2004, Vol. 84. 146.

8. Boltenko E.A., Tarasevich S.E., Shpakovsky A.A. Method for calculating the heat transfer crisis in the region of the dispersed-annular regime on the heat-releasing surfaces of a fuel element with two-sided heat removal / Thermal processes in technology, 2010, vol. 2, no. 6 p. 256-262.

9. Shpakovsky A.A. Development of a method for calculating the thermal-hydraulic characteristics of fuel assemblies with tubular fuel elements. Dissertation for the degree of candidate of technical sciences. Kazan 2014

10. E.A. Boltenko Heat Transfer Crisis and Fluid Distribution in Steam-Generating Channels.- M.: Raduga, 2015. 280 p.

Claims (1)

A method for increasing critical heat fluxes in a fuel assembly with tubular fuel elements, which consists in supplying a coolant to the inlet of the fuel assembly, distributing the coolant between the inter-fuel space and the internal cavities of the fuel elements (fuel elements), passing the coolant along the convex and concave heat-releasing surfaces of the fuel elements, and releasing heat in fuel elements, redistribute the released heat over the thickness of the fuel elements and remove the generated heat due to heat removal from the concave and convex heat-releasing surfaces, characterized in that the redistribution and removal of heat from the heat-removing surfaces is carried out due to the fact that the flow entering the inter-fuel space is preliminarily divided by at least two flows, they are fed to the convex heat-releasing surface of the fuel elements, the flow (upper) remote from the convex surface is swirled, the transit flow flowing along the convex heat-releasing surface of the fuel elements and the swirling flow are interacted, the flows formed by swirlers and flows are interacted, passing along the axis in the gaps between the swirling devices, the interaction of the swirling flow that occurs when the coolant passes through the holes in the wall of the fuel element and the transit flow flowing along the convex heat-releasing surface of the fuel element is carried out.

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